Draining the Swampland

TL;DR

The paper analyzes the swampland criteria $C1: \frac{M_{pl}|\nabla V|}{V} \gtrsim c \sim 1$ and $C2: \Delta\phi \lesssim M_{pl}$ and their implications for inflation and the cosmological constant. It argues that strictly single-field realizations are under significant tension with observations, but introducing a second light scalar, the drainon, can reconcile these criteria in a realistic multi-field framework. The drainon can act as a coherently oscillating field after inflation (potentially as a dark matter candidate) and, with suitable potentials (including logarithmic forms), contribute the necessary gradient without dominating the energy density during inflation. Concrete string-inspired realizations (such as fluxbrane inflation) illustrate how drainon-like fields can realize the required dynamics, suggesting that standard inflation and $\Lambda$CDM can be preserved if swampland constraints are addressed via a drainon rather than discarded.

Abstract

We discuss some implications of the recently suggested Swampland conjecture $\frac{|\nabla V|}{V}\gtrsim c\sim 1$, together with a previous one $Δφ\lesssim 1$. We list some implications for particle phenomenology and the Early Universe. The most intriguing implication of the conjecture could be a significant shift in allowed inflationary models, if not ruling out slow-roll (single field) inflation altogether. The tension of inflation and the conjecture does not only regard the amplitude of the tensor spectrum, but also its tilt, as $c\gtrsim 1$ implies both a yet unobserved tensor to scalar ratio, and an enhancement of the observed scalar power spectrum on large scales in discord with current data that favors a suppression on these scales. Scalar fields are abundant in theories of quantum gravity. Considering a second scalar field, its dynamics are dictated by the relation between its mass, $m$, and the Hubble parameter, $H$, at different epochs in the history of the Universe. This scalar field, a drainon, fulfills the conjecture draining up the swampland. For Inflation, this drainon requires a modest hierarchy compared to the inflaton. For the rest of the thermal history of the Universe, the drainon can be a coherently oscillating scalar field strengthening the case of Dark Matter candidates of that sort.